The library now adds wrappers around the buddy library's sign function. This is
made possible by extracting any private keys from the json web key list.
Specs are added for the new private key functionality.
The resolve-key function is now private, and new functions resolve-public-key
and resolve-private-key has taken its place.
Signed-off-by: Snorre Magnus Davøen <firstname.lastname@example.org>